The present invention relates to corneal implants and methods for producing such implants. In particular, the invention relates to corneal implants structured to be surgically attached in or on the cornea of a mammalian eye which include a hydrogel composition and which have enhanced biostability.
The cornea comprises five layers, including an outer layer of epithelial cells, Bowman's membrane immediately posterior of the cells, the stroma immediately posterior of Bowman's membrane, Descemet's membrane immediately posterior of the stroma and the endothelium immediately posterior of Descemet's membrane. A number of surgical operations involve implanting a corrective lens structure into or onto one or more of these corneal components. For example, in one form of eye surgery, the layer of epithelial cells is removed and a corrective lens structure is placed and secured at the location where the cells were removed. In another form of eye surgery, the layer of epithelial cells is removed and then a wedge-shaped annulus from Bowman's membrane and the underlying stroma is removed. An incision is then made from the posterior end of the resulting groove radially outwardly in an annual zone to define a flap. A corrective lens structure is attached by inserting the wing of the lens structure beneath the corneal flap and fixing, e.g., suturing, it in place. In addition, a corrective lens structure can be placed entirely within the stroma. This surgical procedure involves making an incision in the cornea to gain access to the stroma and also involves disrupting the stroma by placing a lens structure therein.
In each of these surgical procedures, it is highly desirable, even necessary, for the long term viability of such lens onlays or implants that the cornea, e.g., the epithelial cells and/or stromal keratocytes, grow onto the lens structure and/or adhere to the lens structure. Achieving such growth and adhesion has been one substantial problem inhibiting the use of such corneal onlay and implant procedures.
Hydrogel compositions have been suggested for use as materials of construction for corneal implants. As used herein, the term "hydrogel composition" refers to a composition including a material, for example, a polymeric material, and sufficient amount of water to cause the composition to swell relative to the anhydrous material. Such hydrogel compositions often include at least about 38% by weight of water, and may include as much as about 60% or about 80% or about 90% or more by weight of water. One concern of using hydrogel compositions as corneal implants is the relative inability of such compositions to support the growth and/or adhesion of epithelial cells on and/or to the implants.
One hydrogel composition that has been suggested for use in corneal implants is a hydrogel of water and collagen. Such a hydrogel composition has a sufficient nutrient, for example, glucose, diffusivity, and cytophilicity, that is the ability to support at least one of epithelial cell growth and epithelial cell adhesion, to be useful in corneal implants. One problem which has become apparent with such collagen hydrogels is a substantial susceptibility to being degraded by enzymes present in the eye. This is a significant concern since the corneal implant is surgically attached to the eye and is intended for long term, even permanent, placement in the eye.
Therefore, it would be advantageous to provide hydrogel compositions for use in corneal implants which have many, if not all, of the beneficial properties of hydrogels of collagen and water and, in addition, have enhanced biostability relative to such collagen/water hydrogels.